Note: To check for DH param we should check the dhP parameter which is actually in bytes and we multiply it by 8 and convert it to bits (128 bytes = 1024 bits OR 512 bytes = 4096 bits)In Wireshark check for server key exchange packet to get the information for DH param.

Conclusion: Your server is configured for different DHParam i.e 4096 and 1024, NoSNI, and SNI respectively. It is not a bug in SSLLabs.

Note: To check for DH param we should check the dhP parameter which is actually in bytes and we multiply it by 8 and convert it to bits (128 bytes = 1024 bits OR 512 bytes = 4096 bits)In Wireshark check for server key exchange packet to get the information for DH param.

Conclusion: Your server is configured for different DHParam i.e 4096 and 1024, NoSNI, and SNI respectively. It is not a bug in SSLLabs.

I haven't commented on the usage of EC or RSA certificates. Usage of a dual certificate won't cause any issue as reported by you. It is more to do with your configuration of SNI and NoSNI.Please use Wireshark and OpenSSL to confirm the same as shared in the above post.